Human birth defects can be caused by disruption of molecular pathways that regulate cell shape and movements. Cell shape changes and movements are responsible for helping to establish and separate the three primary germ layers at gastrulation, for reification of these germ layers into different tissue types, and for combinin tissues into organs with unique internal architecture and anatomical relationships. Multiple signaling pathways contribute to cell shape and movement, including the Wnt Planar Cell Polarity (PCP) pathway upstream of convergent-extension (CE) movements in vertebrates. In the prior funded period we used an engineered knock-out at the mouse Dapper/Frodo (Dact1) locus to explore how its loss causes a spectrum of developmental defects in mice resembling a major spectrum of birth defects. Through this project we discovered that Dact1 regulates Van Gogh-like (Vangl) transmembrane proteins central to PCP during Epithelial-Mesenchymal Transition (EMT) at the Primitive Streak (PS) during gastrulation. We have now engineered novel knock-out mouse lines for the two remaining Dact family members in mammals (Dact2 and Dact3) as well as for Sestd1, which encodes a Dact-interacting protein identified in my lab. These animals very closely phenocopy Dact1 mutants, strongly supporting the hypothesis that major developmental functions of Sestd1 are allied closely with Dact1. Intriguingly, Sestd1 has previously been shown to help regulate calcium flux through TRPC channel proteins; moreover its closest molecular relative has been implicated in transmembrane protein trafficking. This agrees with evidence suggesting that the molecular pathway emerging from work in my lab acts via localization and/or levels of a subset of transmembrane proteins at the intersection of several important signaling cascades. In the renewed funding period I propose to address these Specific Aims: (1) Do Sestd1 & Dact1 form a genetic pathway with Scribble, Celsr1 & Vangl2 during EMT at the PS; with Dvl2 during CE; and with TRPC4/5 during calcium signaling? (2) Are both the PCP & Calcium pathways biochemically disrupted in posterior embryonic tissues of these mutants? (3) Are membrane localization & levels of pathway components linked to PCP & Calcium signaling in affected embryonic tissues? As in the prior funded period, the research strategy draws on deep mouse genetic, embryonic, cell biologic and biochemical tools custom-designed and assembled in my laboratory expressly to explore this pathway.